Water-absorbent resin has been widely used in various fields, including hygienic articles, such as disposable diapers and sanitary napkins; agricultural and horticultural materials, such as water-retaining agents and soil conditioners; and industrial materials, such as water-blocking agents and agents for preventing dew condensation.
Known types of water-absorbent resin include, for example, hydrolysates of starch-acrylonitrile graft copolymers, neutralized products of starch-acrylate graft copolymers, saponified products of vinyl acetate-acrylic ester copolymers, and crosslinked polymers of partially neutralized acrylic acid compound.
Desired properties for water-absorbent resin include, for example, the absorption properties, such as the amount of water absorption and the water-absorption rate, and a particle size that is optimal for the use of the water-absorbent resin. In particular, for hygienic articles, such as disposable diapers and sanitary napkins, it is necessary to use a water-absorbent resin having a small particle size and a narrow particle size distribution in order to exhibit satisfactory absorption properties, such as the liquid permeation time, re-wet, and dispersibility.
Examples of known methods for producing a water-absorbent resin include an aqueous solution polymerization method and a reversed suspension polymerization method. In the reversed suspension polymerization method, the use of a surfactant is necessary. A method in which a fatty acid sorbitan ester, a fatty acid sorbitol ester, or the like is used as the surfactant is known as a technique that focuses on the surfactant (Patent Literature 1). However, this method has drawbacks; for example, a stable production of a water-absorbent resin having a small particle size is difficult and intense adhesion of resin to the wall surface of polymerization equipment during production is observed.
Another production method, in which a specific fatty acid sucrose ester is used as a surfactant, is also known (Patent Literature 2). This method shows some improvement in productivity since the adhesion of resin to the wall surface of the polymerization equipment during production is prevented; however, there is room for further improvement in this method since it has problems, such as a large particle size, a wide particle size distribution, etc., in the resulting water-absorbent resin.